1. Field of the Invention
This invention relates generally to a method of forming a servo pattern on a rigid magnetic recording disk using a contact magnetic transfer (CMT) template.
2. Description of the Related Art
Conventional magnetic recording hard disk drives use horizontal or longitudinal recording, i.e., the magnetized regions that define the magnetically recorded data bits are oriented in the plane of the recording layer on the hard disk. Perpendicular magnetic recording, wherein the magnetized regions are oriented perpendicular to the plane of the recording layer, has also been suggested for hard disk drives. Hard disk drives use fixed, pre-recorded servo patterns to position the recording heads to the desired track and record location on the disks.
The conventional method of generating the servo pattern is by “servo-writing” the pattern on a track-by-track basis, either with a special write head and servo-writer or with the production head in the disk drive. Because this is a time-consuming and therefore expensive process, contact magnetic duplication or transfer (CMT), sometimes referred to as magnetic printing, has been proposed as a method of instantaneous recording of the servo pattern. In the CMT method a “master” disk or template is used that contains regions or islands of soft (low-coercivity) magnetic material in a pattern corresponding to the servo pattern that is to be transferred to the magnetic recording disk (the “slave” disk). As shown in FIG. 1A, the recording layer of the slave disk is first exposed to a uniform magnetic field (i.e., “DC” magnetized) with a magnet 1 that applies an in-plane horizontal (longitudinal) magnetic field in a first direction 2 across a gap 3 between the magnet's poles. This field is typically applied by rotating the slave disk past the fixed magnet 1 so that the slave disk becomes magnetized in a first circumferential direction. As shown in FIG. 1B, a master template supported on a carrier is then pressed into contact with the DC-magnetized slave disk, and a second horizontal DC magnetic field is applied by magnet 1 in the direction 4 opposite to the direction 2 of the first DC magnetization. This produces a magnetization pattern on the slave disk because the first magnetization on the slave disk is shielded from the second DC field in the regions facing magnetic islands 5 of soft magnetic material on the master template, and the first magnetization on the slave disk is reversed in the regions facing the openings 6 on the template (the nonmagnetic regions between the soft magnetic islands 5 on the master template). The reversed magnetizations are shown by arrows 7 in FIG. 1B. When the field is applied in the direction 4, the magnetic islands 5 become magnetized in the same direction 4, which generates magnetic fields that oppose the applied field in the recording layer regions facing the islands 5, so that there is essentially no field applied to the regions of the recording layer facing the islands. The magnetic field applied to the slave disk beneath the openings 6 is enhanced by the dipole fields 8 in the soft magnetic islands 5 adjacent the openings 6, because in the presence of the field from the magnet these islands generate their own fields. CMT was first proposed for generating servo patterns in longitudinal magnetic recording media in U.S. Pat. No. 3,869,711.
The CMT master template is typically a rigid substrate or a rigid substrate with a plastic film formed on it. These types of master templates have been described in U.S. Pat. Nos. 6,347,016 B1 and 6,433,944 B1; and by Ishida, T. et al., “Magnetic Printing Technology-Application to HDD”, IEEE Transactions on Magnetics, Vol 39, No. 2, March 2003, pp 628–632. In U.S. Pat. No. 6,798,590 B2, assigned to the same assignee as this application, a flexible plastic master template is used in a CMT method that uses a differential gas pressure to press the pattern of magnetic islands against the slave disk.
The Ishida et al. paper has also suggested that the same CMT method for longitudinal magnetic recording disks, wherein a longitudinal magnetic field is applied in the plane of the recording layer as shown in FIGS. 1A–1B, can also be applied to transfer servo patterns to perpendicular magnetic recording disks. U.S. Pat. No. 6,791,774 B1, assigned to the same assignee as this application, describes a CMT template and method for forming servo patterns in perpendicular magnetic recording disks.
In the prior art CMT method, the magnetic islands on the template, which have become magnetized in the desired direction during the application of the applied magnetic field, may not revert to their demagnetized state when the applied field is removed but may retain a residual or remanent magnetization predominantly along their easy axes of magnetization. This residual magnetization of the islands may partially erase the previously transferred magnetic pattern from the slave disk, resulting in a reduction in the signal-to-noise ratio of the magnetically transferred servo pattern. This effect is more pronounced when the magnetic islands are of very small dimensions because the uniaxial magnetic anisotropy induced by the shape of the islands (shape anisotropy) causes the islands to remain magnetized.
What is needed is a CMT method that uses a template that has magnetic islands that readily saturate to produce strong magnetization in the presence of the applied magnetic field, but that exhibit virtually no remanent magnetization in the absence of the applied field.